Conventionally, in a vehicular control technique, a by-wire system for electrically controlling an actuator of changing a vehicle state with a by-wire control circuit according to an instruction from a driver of a vehicle is well known. For example, Patent document No. 1 teaches a shift-by-wire system for switching a shift lever according to an instruction of the driver. In the system, the operation of the actuator having a brushless motor is controlled so that the shift range is changed by rotatably driving a detent plate in a shift range switching device.
In the shift-by-wire system according to Patent document No. 1, based on a counting value of a pulse signal output from an encoder at every predetermined angle in synchronization with a rotation of the motor, the shift range is switched to a target range by rotating the motor to reach a target rotation position, which corresponds to the target range. Further, in the shift-by-wire system according to Patent document No. 1, before starting a switching control of the shift range, i.e., before starting a normal driving control of the motor, the system rotates the motor until the detent plate stops at a limitation position in a movable range so that a standard position learning control for learning a standard position of the motor is performed. When the system learns the standard position, the limitation position coincides with the standard position of the motor. Thus, after that, the system can perform the normal driving control for rotating the motor toward the target rotation position.
Here, when the standard position learning control is executed in a system for controlling a current flowing through each phase of a winding of the motor according to a duty ratio corresponding to a voltage, even if the voltage is same, the current flowing through each phase of a winding of the motor may change according to a change of a winding resistance caused by temperature change and/or chronological change, so that the torque of the motor is changed. Thus, the accuracy of the standard position learning control may be reduced. This view point is not considered in the system according to Patent document No. 1. Thus, in the system according to Patent document No. 1, the accuracy of the standard position learning control may be reduced according to the condition change such as the temperature change or the chronological change.
When the standard position learning control is performed, the current flowing through the windings is limited so as to reach the sum of current flowing through each phase of the winding of the motor to be a predetermined current limitation value. Thus, the change of the current in each phase caused by the temperature change or the chronological change is restricted, so that the accuracy of the standard position learning control is improved even if the condition is changed. However, when the system includes a physical circuit for detecting the sum of the current flowing through each phase of the winding of the motor, and/or when the system includes a physical circuit for restricting the current flowing through the windings, if the circuit is damaged, the standard position may not be learned properly.    (Patent document No. 1) JP-2004-308752-A corresponding to US 2006/0207373 A1